1. Field of the Invention
The present invention relates to a plunger for a syringe which is adapted to aspirate or eject a quantity of liquid medicament into or from a barrel of the syringe.
2. Description of the Related Art
In general, a plunger for a syringe is composed of a piston which is slidably inserted into a barrel and a rod which is screwed in a threaded hole (female hole) formed at the center of the piston. The entirety of the piston is usually made of rubber and is coated at its outer surface with silicone oil to reduce sliding resistance. In recent years, harmful influence of the silicone oil on humans has come into question. To solve this problem, the assignee of the present application has proposed a laminated piston which is made of a rubber piston having a laminated layer. The rubber piston is coated at its outer surface with the laminated layer made of a tetrafluoroethylene resin film, an ethylene-tetrafluoroethylene resin film, or a ultra high molecular weight polyethylene resin film.
However, the laminated piston exhibits a higher sliding resistance to the barrel than the conventional rubber piston coated with silicone oil. When in use, the laminated piston is pushed in the barrel (moved inward) to eject liquid medicament from the barrel, and is drawn (moved outward) to aspirate the liquid medicament into the barrel. In conventional products, when the rod (laminated piston) is drawn (moved outward), there is a possibility that the rod disengages or detaches from the laminated piston, due to the large sliding resistance between the laminated piston and the barrel. This is due to deformation of the threaded hole of the soft laminated piston in which the rod is screwed.